Modern ATX systems are already pretty close to 12VO, all of the non-trivial loads use point-of-load regulators powered from 12V, 12VO is nothing more than the final stretch to ditch legacy cables.
What component in your PC works directly with 3.3V or 5V? Practically none since virtually all modern chips are sub-2V so even if an SSD, PCIe card or USB device draws current from 5V, that 5V is only feeding another point-of-load regulator that takes it down to 0.9/1/1.1/1.2/1.5/etc. volts. If you closely examined your motherboard, you will find well over a dozen DC-DC converters for all sorts of support circuitry. Take a look at a HDD, you will find 4-5 more DC-DC converters. Look at an SSD, 3-4 more on there. GPU? Around a dozen for all of the support rails responsible for making the PCIe, GDDR, display outputs, etc. work besides the obvious Vcore and Vgddr ones. The supply rail count goes up very quickly with those highly-integrated multi-rail controllers where low-power rails can have little more than a 0402 inductor and capacitor on it. One of those and your DC-DC converter total can go up by 5-10. All in all, your system may already contain around 100 DC-DC converters.
Nearly everything uses point-of-load DC-DC converters. Even DDR5 brings 12V DC-DC converters on-DIMM for improved voltage regulation performance and efficiency, there goes 3-4 extra DC-DC converters (Vccio, Vterm, Vpll, Vram, possibly more) per DIMM.
If there was a time to question the reliability of reasonably well made DC-DC converters, I believe we are way beyond that.